This invention relates to a print element for use with a printer having a multiplicity of type members arranged in a plurality of layers.
Generally, in a printer such as a serial impact printer, a print element is used. The print element may be of different types. FIG. 1 shows a print element in the form of a disc type print wheel generally referred to as a daisy wheel including a plurality of spokes 2 extending radially from a center hub and supporting at their forward end portions type members 5 and 6 located on support members 3 and 4 respectively and arranged in a plurality of circumferences. FIG. 2 shows a bowl type print wheel including a plurality of spokes 2 arranged around a cylindrical or frustoconical surface and supporting at their forward end portions a multiplicity of type members 5 and 6 supported on the support members 3 and 4 respectively. Said daisy wheel and bowl type print wheel are generally referred to as petal type print wheels. In many of these type wheels, type members are arranged in two layers on one spoke in order to contain many characters necessary for printing. FIGS. 1 and 2 show examples of this arrangement of the type members. When the type members are disposed in a plurality of layers, the print wheel is mounted through a universal joint on the drive shaft of a selection motor in the case of the disc type print wheel shown in FIG. 1. In the case of the bowl type print wheel shown in FIG. 2, the wheel is mounted slidably on the drive shaft and the print wheel is shifted in the vertical direction by a print signal, to bring one of the type members 5 and 6 to a predetermined print position as shown in FIG. 3 and give an impact to the back of the support members 3 or 4 by a print hammer 10. Thus the type member 5 or 6 strikes through an inked ribbon 9 a sheet of paper 8 wound on a platen 7 to carry out printing.
In this case, the adjacent type members are preferably located as close as possible to each other to reduce the radius of the print wheel, in order to make it possible for the print wheel to rotate at high speed by reducing the moment of inertia of the print wheel as a whole. In the print wheel of the type in which the type members are arranged in two layers in each of the spokes, it is necessary that the spacing interval between the print members of the two layers be minimized so as to reduce the time required for the shifting of the print wheel in a vertical direction and simplify the shifting mechanism by reducing the distance covered by the vertical shifting movement of the type wheel.
The type members 4 and 5 have their surfaces concavely curved as shown in FIG. 4 to conform to the shape of the outer peripheral surface of a platen 7. Generally, the height a of the central portion of each of the type members 5 and 6 from the surface of one of the support members 3 and 4 has had the same value for all the type members 5 and 6. Thus in the case of a print wheel having the two type members 5 and 6 on each of the spokes 2, the central portions of the two type members 5 and 6 naturally have the same value a. When this is the case, there would be the possibilities that as printing is carried out on the paper 8 by the type member 6 as shown in FIG. 5, the spoke 2 would be flexed toward the platen 7 in the forward end portion thereof as seen from the type member 6 by inertia at the time an impact is given to the type member 6 and a lower end portion 5a of the type member 5 would also strike the paper 8 if the distance between the two type members 5 and 6 were reduced. This would result in the spoiling of the paper 8 by ghost printing.
The position in which a type member is struck by the print hammer 10 is constant at all times because a hammer guide 11 (see FIG. 3) is fixed, and impact is given by the print hammer 10 to the center of the back of each of the support member 3 and 4. The type members 5 and 6 on the surfaces of the support members 3 and 4 respectively differ from each other in position or size with respect to the center at which the impact is given to the back of each support member, depending on whether the type members are characters or symbols. Assume that, as shown in FIG. 6, a characters, such as 1/4, which is more elongated downwardly than ordinary capital letters, such as A, B and C, is supported on the support member 3 as the type member 5, and the character 6 supported on the support member 4 is a type member, such as ",", which is disposed in a position below the center of the support member 4 at which impact is given. In the case of a spoke which supports type members of this type of combination, no problems are raised when printing is carried out by the type member 5 as shown in FIG. 7 because the distance l.sub.1 between an impact center .phi..sub.1 and an upper end 6b of the type member 6 and hence the gap .delta..sub.1 between the upper end 6b of the type member 6 and the platen 7 is large. However, when printing is carried out by the type member 6 as shown in FIG. 8, the distance l.sub.2 between an impact center .phi..sub.2 and a lower end 5a of the type member 5 is small and hence the gap .delta..sub.2 between the lower end 5a of the type member 5 and the platen 7 is small. Thus even if the support members 3 and 4 of the two type members 5 and 6 are formed integrally with each other, ghost printing tends to be performed by the lower end portion 5b of the type member 5.
The characters and symbols that extend downwardly more than the ordinary capital letters include, besides the aforesaid "1/4", "2/4", "1/2", "3/4", "/", "(",")", "{","}", ".delta.", " ", "#", etc. The symbols whose type faces deviate upwardly or downwardly from the impact center include, besides the aforesaid ",", ".", "'", " / ", " ", """, ".about.", " ", etc.